This invention relates to a power supply system for an electronic apparatus incorporating a volatile memory and, particularly, to a power supply system suitable for use with a data processing system.
A conventional power supply circuit for a volatile memory is disclosed in Japanese Patent Unexamined Publication No. 57-23123. This prior art circuit arrangement is intended to automatically switch the supply of electric power from the main power supply to the battery in response to the turn-off operation of the main power supply. However, this system needs a battery of large capacity depending on the expected turn-off duration of the main a.c. power supply, since the battery is used continuously during the "power-off" state regardless of the presence or absence of the a.c. power input.
Another prior art technique disclosed in Japanese Patent Unexamined Publication No. 56-168246 is the use of a battery in response to the detection of a.c. power failure. However, this system operates in response to two conditions, i.e., the output voltage of a main power supply has built up or a.c. power failure exists, and there is no description in the publication on the "power-off" operation of the main power supply. Accordingly, when the main power supply is turned off, battery back-up is started as in the case of power failure, resulting in over-discharging of the battery.
Another technique similar to that of the above patent publication 56-168246 is discribed in Japanese Patent Unexamined Publication No. 59-24323. This publication does not describe the "power-off" operation of the main power unit and power failure and power-off can not be distinguished, resulting in over-discharging of the battery.
Still another technique disclosed in Japanese Patent Unexamined Publication No. 56-116134 is intended to operate such that the battery back-up for the volatile memory is suspended upon expiration of a certain time length after the state of power failure has been detected so as to prevent over-discharging of the battery. Also in this publication, no description is given for the power-off operation for the main power supply, and the system treats the power failure in the power-off state in the same way as the power failure in the power-on state, resulting in over-discharging of the battery.